As a prey species, horses have evolved to cope with short periods of rapid eye movement (REM) sleep and are known for their ability to sleep, or at least doze, (Non-REM sleep, NREM) while standing up. They still have to lie down for REM sleep which is only possible during recumbency due to the lack of muscle tone that occurs during this sleep phase.
In research conducted at Aberystwyth University, Linda Greening and others investigated the effect of two different light regimes, and two different depths of straw bedding on sleep patterns in horses. They also looked at whether the differences affected the horses’ response to a memory test – basically remembering which of four buckets placed around an arena contained a feed reward.
In a report in Applied Animal Behaviour Science, the authors describe how ten adult riding school horses were randomly assigned to two equal-sized groups for the five-week experiment. Each group experienced a combination of one of two light conditions (lights on or off overnight), and one of two bedding depth treatments (straw bed 15 cm or 5cm deep) for six days.
The authors found that the duration of sleep stages was affected by both the depth of bedding and lighting conditions. Under sub-optimal conditions, horses tended to spend significantly less time recumbent for both NREM and REM sleep. This was compensated for by increased time in standing NREM sleep.
Sub-optimal lighting and bedding conditions did not have a significant effect on performance during the spatial memory test. The authors suggest that, “a more cognitively demanding task may be required to differentiate the performance-related consequences of different levels of sleep duration and quality.”
For more details see:
The effect of altering routine husbandry factors on sleep duration and memory consolidation in the horse
Linda Greening, Josh Downing, Daniella Amiouny, Line Lekang, Sebastian McBride
Applied Animal Behaviour Science
Volume 236, March 2021, 105229